Wax compositions for impregnating paperboard



United States Patent ABSTRACT OF THE DISCLOSURE The wet crush strength of paperboard is improved by impregnating it with a composition containing 45-70% paraflin wax,25-50% microcrystalline wax, 1-10% ethylene-ethyl acrylate copolymer and 1-10% Fischer- Tropsch wax.

In one aspect this invention relates to novel wax compositions particularly suitable for impregnating paperboard in order to improve the wet crush strength thereof. In another aspect the invention relates to paperboard impregnated with either of these wax compositions as an article of manufacture. The invention also embraces methods of improving the wet crush strength of paperboard which involve impregnating the latter with either of the novel wax compositions.

In the short-distance transportation of certain types of foods, e.g., chickens, fish, etc., the food is often packed in ice in the shipping container. Since most of the ice melts during transportation, the material from which the container is fabricated must have a high wet crush strength so that the containers can be stacked without danger of collapsing. Ideally the wet crush strength would be at least as high as the dry strength. Heretofore wood containers have probably been the most widely used since they have adequate wet crush strength. Although corrugated paperboard containers are probably the most widely used containers if all packaging applications are considered, they are seldom used where the goods are packed in ice because paperboard has almost no wet crush strength.

Improvements in the wet crush strength of paperboard have been made in the past by impregnating the board with wax, either wax alone or wax containing one or more additives. See, e.g., US. Patents 2,967,781, 2,967,116, and 3,085,026. However, the manufacturers of corrugated board want paperboard having considerably more wet crush strength than can be obtained by the prior art techniques. These manufacturers have indicated a desire for paperboard having two characteristics. One characteristic is that the crush strength of the treated, i.e., wax impregnated, paperboard after the latter has been submerged in liquid water for 24 hours should be at least 90% of, preferably equal to or greater than, the crush strength of the unwaxed, unsubmerged board. In other words, impregnation of the board with wax should permit immersion of the board in water without loss of more than preferably without any loss, of its crush strength. Stated in still another manner the strength retention of the board after submergence in liquid Water for 24 hours is at least 90% and is preferably at least 100%. The test used to measure crush strength is described in detail subsequently. I

The other characteristic is that the crush strength property described above must be obtained with a wax pick-up of not more than 55%. A wax pick-up of 55% means that the amount of wax in the impregnated board is 55% by weight of the unimpregnated board, which is equivalent to 36.7% based on the total Weight of the impregnated board. Another manner of expressing this "ice second characteristic is that the board has a strength retention of at least at a wax pick-up not exceeding 55%. It will be noted that these two characteristics are extremely severe. Although there are a few wax compositions which will yield strength retentions of 90- the wax pick-up required is substantially higher than 55%. Secondly, the use of a 24-hour submergence period in the water immersion test is substantially longer than the period normally employed. For example, 'in US. 3,085,026 a two-hour submergence period is used.

I have now found two wax compositions either of which can be used to prepare paperboard having the characteristics described above. The compositions are superior to any wax compositions known heretofore for improving the wet crush strength of paperboard.

My two novel compositions, referred to hereinafter as Composition A and Composition B, will nowbe described in detail after which their remarkable effectiveness in improving the wet crush strength of paperboard will be shown by several examples.

The ingredients of Composition A and the operable and preferred amount of each ingredient are as follows. All percentages herein are by Weight of the total composition.

The phrase operable amount means that so long as each ingredient is present in the amount specified the strength retention of the impregnated paperboard will be at least 90% at a wax pick-up of 55%. Best results are obtained, however, when each ingredient is present in the preferred amount in which case the strength retention will usually be at least 100% at a wax pick-up of 55%.

Parafiin and microcrystalline waxes are well known articles of commerce and can be obtained from petroleum by well known techniques. Paraffin waxes normally have melting points in the range of -1'65 F. (ASTM D87-57), penetrations in the range of 5-25 dmm. (ASTM D1321-61T, 100 g., 5 sec), and viscosities in the range of 30-50 S.U.S. (ASTM D446-53). The melting points, penetrations, and viscosities of the microcrystalline waxes are normally -200 F., 5-25 dmm., and 60-100 S.U.S., the tests by which these properties are determined being those used for parafiin wax except for melting point which is determined by ASTM Dl27-60. The paraffin and microcrystalline waxes preferably have melting points of l35-155 F. and -180 F. respec tively.

The polyethylene suitable for use in Composition A should be a wax grade polyethylene. Accordingly it should have a molecular weight between 2000 and about 20,- 000 (by solution viscosity), preferably 3,000-15,000. Although not critical, polyethylenes suitable for the present purpose will normally have a density of 0.87-0.97 gms./cc. (ASTM D1505-57T).

The ethylene-ethyl acrylate polymer is made by copolymerizing ethylene and ethyl acrylate according to well known techniques. For example, suitable copolymers can be prepared by the procedures described in US. Patent 2,200,429. To be suitable for use in Composition A the copolymer should contain 10-40% ethyl acrylate, preferably 15-35%. Although not critical the copolymer will normally have a melt index of 01-100 (gms./ 10 min.) and a Vicat Softening Point of 50- 500 F.

The ingredients of Composition B and the operable and preferred amount of each ingredient are as follows.

COMPOSITION B Operable Preferred Ingredient Amount, Amount,

Percent Percent 1. Parallin Wax 4b-70 50-65 2. Microcrystalline Wax -50 -45 3: Ethylene-Ethyl Acrylate Copolymer. 1-10 2-8 4. Fischer-Tropsch Wax 1-10 2 8 commerce and can be prepared by known techniques.

These waxes are made by reacting hydrogen and carbon monoxide and are characterized by a high percentage, e.g., over 80%, of normal paraflins. Fischer-Tropsch waxes normally have, and should have for the present purpose, melting points of 180250 F. Preferably the melting point is 205 -235 F.

The compositions of the invention are, of course, homogeneous blends of the various ingredients. They can be prepared by heating the paraffin wax to, say, 200 F. at which temperature it will be molten. The other three ingredients are then added and the mixture stirred until it is homogeneous.

The amount of the composition applied to the paperboard will vary depending upon use conditions, etc. It was pointed out previously that the superior properties of our compositions are evidenced by the fact that they yield strength retentions of at least 90%, usually at least 100%, at wax pick-ups as low as As will be evi dent from some of the examples infra, strength retentions of at least 90100% can often be obtained at wax pickups of less than 55%. In any event, regardless of the wax pick-up the compositions will yield higher strength retentions than are obtained with conventional compositions at the same wax pick-up. The amount of the present compositions applied to the paperboard will normally be equivalent to a wax pick-up of 3055% which is equivalent to 23.1-36.7% based on the total weight of the impregnated board. In most cases the wax content of the impregnated board will be at least 28.6% (a wax pickup of 40%) and will rarely exceed 36.7%.

The test procedure used to determine the effectiveness of the above Wax compositions for improving the strength retention of paperboard is as follows.

The paper stock used is 6" x 6" sheets of brown kraft paper (43 lbs./ 1000 ft. This particular stock is a typical paperboard for making corrugated board.

A 6" x 6" sheet of board is weighed and then impregnated with the wax composition to be evaluated by dipping the board in the molten composition at 190 F. for 15 seconds and then removing the board and allowing it cool to room temperature. The impregnated board is then Weighed and the wax pick-up calculated. The amount of wax composition in the board is adjusted to the desired level by placing the impregnated board vertically in an oven at 190 F. and allowing excess wax to melt and drain off the board. Since it is extremely important that the board be uniformly impregnated the board is periodically inverted in the oven. After a short period the board is removed and weighed and the wax pick-up is again calculated. This oven draining procedure is then repeated until the desired wax pick-up is obtained.

The impregnated board is then submerged in water at 73 F. for 24 hours after which time it is removed, excess water is wiped therefrom, and five strips /2 x 6" are cut from the board. Each of these 5 strips is then 4 immediately subjected to the Ring Crush Test using the well known Hinde-Dauch Crush Tester. The apparatus employed in this includes a holder which comprises a flat aluminum base containing a circular cutout A" deep by 6" in circumference and a plurality of circular discs A1 thick and of varying diameter. Each of the discs can be placed in the cutout and when this is 'done an annular groove is formed into which the /2 x 6" strip to be tested is inserted in an edgewise position leaving a width of A" protruding above the top of the holder. The disc employed is selected so that the annular groove receives the specimen snugly.

The test is conducted by resting a flat surface on the entire top edge of the specimen and then applying force downward against the surface, i.e., against the specimen edge. The pounds required to crush the specimen is the wet crush strength for that specimen and the average wet crush strength for the five specimens is the wet crush strength of the impregnated paperboard.

Next the crush strength of the original paperboard, i.e., of the paperboard when neither impregnated with wax nor immersed in water, is determined. This crush strength is the dry crush strength of the unimpregnated board. Strength retention (S.R.) is calculated by the formula SR. (percent)= Wet Crush Strength of Impregnatcd Board Dry Crush Strength of Unimpregnated Board As used in this disclosure and claims strength retention is determined using the test procedure and formula described above.

The data in Table I below shows the strength retention of kraft paper of the type described above'when impregnated with two diiferent wax compositions, the wax pickup in each case being 55%. One composition is exemplary of Composition A and the other composition is exemplary of Composition B. The paraffin wax used in each composition has a melting point of 145 F., a penetration at 77 F. of 9 dmm., and a viscosity at 210 F. of 40 S.U.S. The microcrystalline wax used in each composition has a melting point of 170 F., a penetration at 77 F. of 16 dmm., and a'viscosity at 210 F. of S.U.S. The ethylene ethyl acrylate copolymer used in each composition has an ethyl acrylate content of 18%, a melt index of 6, and is known commercially as DQD 6169. The polyethylene used in one composition has a molecular weight of 7000, a density of 0.93, and is known commercially at DYLT. The Fischer-Tropsch wax used in one composition has a melting point of about 215 F. and is known commercially at Parafiint.

TABLE I The data contained in Table I clearly show the remarkable strength retentions which can be obtained by means of my compositions. i

The data in Table II below show the strength retention of compositions which can be described as modifications of those shown in Table I above. Composition 3 is the same as Composition 2 except that the ethylene-ethyl acrylate copolymer is replaced by an ethylene-vinyl acetatc copolymer having a vinyl acetate content of about 25%. The latter type of copolymer is widely used in wax coating compositions. Composition 4 is the same as Composition 1 except that the polyethylene used (known commercially as DYNH-3) has a molecular weight in excess of about 25,000 instead of being 7000.

TABLE II Strength retention By comparing the strength retentions of Compositions 2 and 3 it is apparent that the type of copolymer employed is critical. A comparison of the strength retentions of Compositions 1 and 4 shows that the type of poly ethylene is critical.

I claim:

1. An article of manufacture comprising paperboard impregnated with a wax composition containing 45-70% of a parafiin wax having a melting point of 120-165 E, 25-50% of a microcrystalline wax having a melting point of l40-200 F., 1-10% of the copolymer of ethylene and ethyl acrylate containing -40% ethyl acrylatc, and 1-10% of a Fischer-Tropsch wax having a melting point of ISO-250 F., said impregnated paperboard having a strength retention of at least 90% after submergence in liquid water for 24 hours when said wax composition constitutes 36.7% of said impregnated paperboard, all percentages being by weight.

2. Article according to claim 1 wherein in said wax 6 composition the amount of paraifin wax is -65%, the amount of microcrystalline wax is 30-45 the amount of Fischer-Tropsch wax is 2-8%, the amount of co polymer is 2-8%, the melting point of the parafiin wax is 135-155 F., the melting point of the microcrystalline wax is 16'0-180 F., and the melting point of the Fischer- Tropsch wax is 205-235 F.

3. A wax impregnating composition for paperboard comprising 45-70% of a paraflin wax having a melting point of 165 F; 25-50% of a microcrystalline Wax having a melting point of l40200 F., 1-10% of the copolymer of ethylene and ethyl acrylate containing 10-40% ethyl acrylate, and 110% of a Fischer-Tropsch wax having a melting point of 180-25'0 F.

4. Wax composition according to claim 3 wherein the amount of paraffin wax is 5 0-65%, the amount of microcrystalline wax is 30-45%, the amount of Fischer- Tropsch Wax is 2-8%, the amount of copolymer is 2-8%, the melting point of the paraffin wax is F., the melting point of the microcrystalline wax is -180 F., and the melting point of the Fischer- Tropsch wax is 205 -235 F.

References Cited UNITED STATES PATENTS 2,967,781 1/1961 Jakaitis 106-270 3,085,026 4/1963 Weisgerber et al. 260-285 3,102,040 8/1963 Williams et al. 106-270 3,205,186 9/1965 Zaayenga 260-285 3,245,930 4/1966 McDowell et al. 260-285 FOREIGN PATENTS 634,016 12/1963 Belgium. 1,389,415 1/1965 France. 6,400,903 8/1964 Netherlands.

MORRIS Li-EBMAN, Primary Examiner.

B. AMERNICK, Assistant Examiner. 

